Method of producing sodium beta-alumina single crystals

ABSTRACT

Single crystals of sodium beta alumina are produced from a melt containing NaO and Al2O3 in a gaseous environment containing from about 5,000 to 10,000 parts per million oxygen.

United States Patent 11 1 Yancey Nov. 4, 1975 METHOD OF PRODUCING SODIUM 3,608,050 9/1971 Carman 264/332 B T SINGLE CRYSTALS 3,625,868 12/1971 Grabmaier..

3,715,194 2/1973 Plooster .1 23/305 A Inventor: Paul J. Yan y, Mentor, OhlO 3,795,723 3/1974 Clendenen et al 423/600 Assignee: Union Carbide Corporation, New 3,859,427 1/1975 Francis et al. 1, 423/600 York, NY.

22 Filed; July 2 1974 Primary Examiner-Norman Yudkoff Assistant ExaminerCurtis P. Ribando [21] Appl' N05 492,279 Attorney, Agent, or FirmF. J. McCarthy [52] US. Cl. 23/301 SP; 23/302; 23/305;

423/119; 423/600 57 S C [51] Int. Cl. B01J 17/18 [58] new of Search 23/273 301 305 Single crystals of sodium beta alumina are produced 23/302; 423/119 120, 600 from a melt containing NaO and A1 0 in a gaseous environment containing from about 5,000 to 10,000 [56] References Cited parts per million oxygen.

UNITED STATES PATENTS 3,595,803 7/1971 Du ger 23/305 A 4 Claims, 1 Drawing Figure US. Patent Nov. 4, 1975 METHOD OF PRODUCING SODIUM BETA-ALUMINA SINGLE CRYSTALS The present invention relates to the production of single crystals of sodium beta-alumina, Na O.5Al O to Na O.l lAl O More particularly the present invention is directed to a method for growing single crystals of sodium beta-alumina by the well known Czochralski crystal growth technique.

The Czochralski technique for crystal growth involves heating raw material having a chemical composition corresponding to that of the desired crystal in a refractory metal crucible to the melting point to provide a melt having a chemical composition corresponding to that of the desired single crystal. At the melting point a single crystal seed crystal of composition similar to the desired crystal is dipped into the melt surface and slowly withdrawn upward. This is done in such a way that a meniscus of the melt attaches itself to the seed and is pulled up slightly by its upward motion. The liquid being just at the melting point, even a relatively small temperature decrease can cause solidification of melt on the seed crystal. The liquid which is raised above the surface is cooled slightly and crystallizes out on the tip of the seed forming as a single crystal corresponding to the seed crystal. This crystallized portion in turn draws a meniscus below it as the upward motion continues and the process continues to ultimately provide an elongate single crystal. In this way, long single crystals can be produced as described in US Pat. No. 3,715,194 Plooster and U.S. Pat. No. 3,608,050 to Carmen et al. Sodium beta-alumina has composition in the range of Na O.5Al O to Na O.1 lAl O (1:5 to 1:11 soda to alumina). Basic to the Czochralski technique described above is the preparation of the melt by the melting of the raw materials, e.g. Na O and A1 in proportions of 1:5 and 1:1 1. Previous work (Tyco Laboratories, NASA Final Report on Contract NAS 3-14410) has shown that sodium beta-alumina crystals attempted to be produced in this manner remelted, not in single crystals of sodium beta-alumina but in admixtures of sodium beta-alumina and alpha-alumina, apparently due to continuous loss of Na from the melt by vaporization. At the high temperature required the vapor pressure of Na O is sufficiently high that vaporization occurs under normal conditions and Na is continuously lost from the melt.

It is accordingly an object of the present invention to provide a method for producing single crystals of sodium beta-alumina by the Czochralski technique.

Other objects will be apparent from the following description and claims taken in conjunction with the drawing wherein the single FIGURE of the drawing shows apparatus suitable for the method of the present invention.

A method in accordance with the present invention for producing single crystals of sodium beta'alumina comprises:

i. providing a melt having a composition of Na O.- Al O to Na OJ lAl O in a gaseous environment at a pressure of 760 mm 1' 50% pressure and containing oxygen in an amount of between about 5000 to 15,000 parts per million;

ii. contacting the melt with a seed crystal of sodium beta-alumina and iii. pulling melt material with said seed crystal to form an elongate single crystal of sodium beta-alumina.

1n the practice of the present invention, with reference to the drawing a crucible 10, suitably made of iridium is supported on a mount 12, suitably made of ceramic, such as alumina. Induction heating coil 14 provides heat energy to raise the charge materials (i.e. Na O and Al o lplaced in crucible -10 to the melting point to provide a melt 16 having a chemical composition corresponding to sodium beta-alumina, i.e. Na O.- 5Al O to Na O.1lAl O A' seed crystal 18 of sodium beta-alumina is placed in contact with melt 16 and raised to pull an elongate single crystal mass 20 of sodium beta-alumina. The aforementionedarrangement is enclosed within a vessel 22, suitably formed of glass and a gaseous environment is provided by way of inlet 24. The gaseous environment contains from about 5,000 to 15,000 parts per million (0 by volume), preferably about 10,000 ppm. The remainder of the gaseous environment can be nitrogen, the noble gases or other gases non-reactive with the melt. The gaseous environment exits at outlet 26 and the total pressure within vessel 22 is substantially atmospheric pressure and can range from 760 mm i 50%. In the practice of the present invention, a mixture of Na O and A1 0 e.g. in the form of a fired powder mixture, in the mole proportion of 1:5 to 1:11 is placed in crucible l0 and with a gaseous environment as above described, provided within vessel 22, heating coils 14 are energized to melt the materials in the crucible and provide a melt having a chemical composition of from Na O.5Al O at its melting point on the order of 1970 C. With the aforementioned gaseous environment being maintained, seed crystal 18 is caused to contact melt 16 and is withdrawn as described hereinabove to produce a single crystal 20 of sodium beta-alumina having a chemical composition corresponding to the melt composition. In the practice of the present invention there is essentially no loss of sodium from the melt and the resulting single crystal is entirely sodium beta-alumina. A particular advantage of the present invention is that it can be practiced at atmospheric pressure whereby the need for special equipment is avoided.

The following example will further illustrate the present invention. A

EXAMPLE About grams of fired powder (Na O.l lAl O were placed in an iridium crucible having an inside diameter of 2% inches, a wall thickness of 0.1 inch and a height of 2% inches. The crucible was placed within a seven turn induction heating coil having an ID. of five inches. The crucible stood on a pedestal containing packed zirconia powder while the space between the coil and the crucible was also packed with zirconia powder. This entire apparatus was enclosed in a glass bell jar having an aperture at its top. A nitrogen atmosphere containing about l0,000 ppm oxygen was maintained inside the bell jar. The induction heating coil was energized from a well known R.F. induction heating unit and the power was increased until the induced current in the iridium crucible heated it to a white heat. Conductive heat from the iridium crucible then melted the charge to form a melt. A unicrystalline betaalumina seed rod 0.3 inch diameter was lowered through the aperture in the bell jar until it contacted the surface of the melt. The seed was then withdrawn from the melt at about 0.09 inch per hour for about 30 hours. A final elongated boule having a diameter of 1.8 cm and length of about 7 cm was obtained. This boule 4 ii. contacting the melt with a seed crystal of sodium beta-alumina and iii. pulling melt material with said seed crystal to form an elongate single crystal of sodium beta-alumina. 2. A method in accordance with claim 1 wherein the pressure is substantiallyatmospheric pressure.

3. A method in accordance with claim 1 wherein the gaseous environment is a flow of gas.

4. A method in accordance with claim 1 wherein oxygen is in an amount of about 10,000 parts per million. 

1. A METHOD FOR PRODUCING SINGLE CRYSTALS OF SODIUM BETA ALUMINA NA2O.5AL2O3 TO NA2O.11AL2O3, WHICH COMPRISES I. PROVIDING A MELT HAVING A COMPOSITION OF NA2O.5AL2O3 TO NA2O.11AL2O3 IN A GASEOUS ENVIRONMENT AT A PRESSURE OF 760 MM $ 50% PRESSURE AND CONTAINING OXYGEN IN AN AMOUNT OF BETWEEN ABOUT 5000 TO 15,000 PARTS PER MILLION, II. CONTACTING THE METAL WITH A SEED CRYSTAL OF SODIUM BETAALUMINA AND III. PULLING METAL MATERIAL WITH SAID SEED CRYSTAL TO FORM AN ELONGATE SINGLE SRYSTAL OF SODIUM BETA-ALUMINA.
 2. A method in accordance with claim 1 wherein the pressure is substantially atmospheric pressure.
 3. A method in accordance with claim 1 wherein the gaseous environment is a flow of gas.
 4. A method in accordance with claim 1 wherein oxygen is in an amount of about 10,000 parts per million. 